In recent years in the field of optical communications, optical transmission systems are designed to provide faster data transfer speeds and greater capacities. The core technology of such systems is the wavelength division multiplexing (WDM), a prevalent system that multiplexes optical signals with different wavelengths onto a single optical fiber. To operate a stable communication system using the WDM system, spare light sources are required in case of light source failure. Unfortunately, if the spare light sources are prepared for each multiplexed optical signal, the number of spare light sources increases, resulting in increased costs for maintaining these light sources. To minimize the costs, there is a growing need for tunable light sources that enable a single light source to emit laser light with different wavelengths.
Typical tunable light sources employ a system that changes the temperature of a semiconductor laser to thereby enable tuning of oscillating wavelengths. The tunable oscillating-wavelength range of the semiconductor laser in this system, determined by this system's operating-temperature range, is within about 2-3 nm. Thus, a tunable light source that includes a plurality of semiconductor lasers is often used to broaden the range of emittable wavelengths.
Furthermore, it is required that tunable light sources maintain stable optical-signal wavelengths for a long period of time. To stabilize wavelengths, the wavelength of light emitted from a semiconductor laser is monitored to control the temperature of the semiconductor laser and the like. For this reason, a light source equipped with a wavelength monitor has been developed (see Patent Literature 1: Unexamined Japanese Patent Application Kokai Publication No. 2002-185074 and Patent Literature 2: Unexamined Japanese Patent Application Kokai Publication No. 2012-129259).
Patent Literature 1 describes a wavelength monitor in which a portion of light emitted frontward from a semiconductor laser is extracted by a beam splitter or the like and then directed into an optical filter, such as an etalon, to thereby monitor wavelengths.
Patent Literature 2 describes a wavelength monitor in which light is emitted backward from semiconductor lasers that are provided with irregularly-spaced light-emitting positions, and the light is then directed into an etalon, to thereby monitor wavelengths.
Etalons, which serve as optical filters, have a transmission characteristic that is shifted toward a frequency region depending on a light-incident angle. Thus, to obtain a desired characteristic, the etalons described in Patent Literatures 1 and 2 need to be placed at a precise angular alignment, for example, such that the angular alignment deviation of the etalon is not more than 0.05°.
In particular, in the wavelength monitor of Patent Literature 2, an end semiconductor laser of the arrayed semiconductor lasers has a light-emitting position that is offset from the center axis of a lens. Consequently, a collimated beam after transmitted through the lens propagates at a tilted angle, which increases the angle of beam-incidence on the etalon. Etalons serve as optical filters whose transmittance has a periodic frequency-dependence due to interference involving multiple reflections of light that propagates inside the etalon. Thus, as the angle of beam-incidence on the etalon increases, positional changes of the interfering light in case of the occurrence of angular-variations become greater. This results in a highly-varied transmission-wavelength characteristic of the etalon.